This invention relates to a method and apparatus for producing sound in which an internally generated flow of air is provided, and a valve is employed to modulate the air flow to produce sound waves.
The modulation of air flowing at high pressures to produce high intensity sound is a well known technique. The rotary siren is one example of such a device, in which a rotary valve is used to modulate a flow of air under high pressure. In the case where a random wave form is to be produced, such a voice, music, or other complex sounds, the valve which modulates the pressurized air flow is typically driven by an electromagnetic transducer, such as a voice coil, connected to an electronic audio source.
While devices of the foregoing nature can produce sound at high levels or intensities, i.e., up to ten thousand acoustic watts, they suffer from significant shortcomings. Since all these devices use high pressure air, typically 10 to 50 PSI or more, and are controlled by a single valve that is half open when there is no signal, they produce high noise levels, non-linearities, and sound distortion. Net air flow from the port of the device is also produced. Also, almost all of such devices depend on an acoustic horn to provide most of the acoustic loading. This, in turn, limits operation to frequencies which are high enough where the horn can be of a practical size or length. The lower the frequency, the larger the horn required.
As an example of the above limitation, in order to produce a sound at 3 Hz using the above high pressure devices, the proper horn length would be in the order of 100 feet, and the mouth diameter would have to be in the order of about 90 feet. For this reason, these prior art devices are ill suited to low frequency operation.